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Not receiving mails from some senders due to DNS configAn MTA should be querying the domain after the @ in the email address for MX records and nothing else. If it gets any, it should immediately attempt delivery to one of the lowest MX records. If all MX servers fail to connect or no MX records are found, it should try connecting to the domain itself. The MTA in question is obviously going too far in resolving information, or isn't following the rules for determining what mail server to connect to. There should be nothing wrong with having your domain point to a CNAME - but you need the MX records for email to work.

May21

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Not receiving mails from some senders due to DNS configYou clearly do not understand DNS and the downvote was completely unwarranted. You also edited your question after I posted my answer adding extra information. You also never mention once that you do not have access to your zone file despite clearly mentioning you have changed them.

map www.abc.com to www.t.xyz.com using cNAmeIf your website www.t.xyz.com is sharing an IP address and/or the web server uses the host name to figure out what site to serve, you must also configure the web server to recognize www.abc.com as an alias for www.t.xyz.com.

DKIM failure when using Exim4 as an “edge transport” for Exchange 2010When is Exim doing the DKIM signing? If Exim is acting as a relay, remember that, typically, it will add a header stating that it picked up the message from another server. If the header (or the message at all) is being altered after it's being signed, then the DKIM signature will be off.

Using DNS entries to determine locationOk, so then you want a centralized location system but if someone moves the server and doesn't update the data, then the host will not know its location changed. Sandman4's recommendation of DHCP is a good one if you want something centrally managed. Perhaps you should look at applications that can put together a network topology map based on MAC addresses and using router/bridge information so that you can get an idea of what's located where...

Apr4

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nameservers one domain one VPS and one VMCloudYes, however I wasn't going to confuse the poster - they don't need to know that the WHOIS info is just informative - just that it matters they have the proper nameservers registered as the authoratative nameservers at their registrar and not just updating the NS records for DNS.

Apache Virtualhost entry with Windows hostnameYou shouldn't have to create a virtual host entry for every single possible domain that points to your server just to prevent access. His problem is that Apache, by default, has an allow all access rule. What very few people know to do is set a deny all rule for "/" so that the server by default is locked down unless you explicitly specify access.